You could always delete the entry under socket.namespace.manager.rooms
that contains your socket.
socket.join(socket.id);
console.log(socket.namespace.manager.rooms['/'+socket.id]);
socket.namespace.manager.rooms["/"+socket.id].splice(socket.namespace.manager.rooms["/"+socket.id].indexOf(socket.id),1);
console.log(socket.namespace.manager.rooms['/'+socket.id]);
This returns
[ '36_IcxqkWgBgLfC6p3Yj' ]
[]